Hierarchy is also reflected in the degree-degree correlation through the conditional probability \(P(k|k^{\prime })\) , i.e. the probability that a vertex of degree \(k^{\prime }\)  is linked to a vertex of degree \(k\) . This function is difficult to measure, due to statistical fluctuations, and it is usually substituted by the Average Nearest Neighbors Degree (ANND), defined as \(\left\langle k_{nn}(k)\right\rangle =\sum _{k^{\prime }}k^{\prime }P(k^{\prime }|k)\)  [1]}. For independent networks this quantity would result independent of \(k\) . Figure REF  (inset) reports the ANND for the WTW and IMN, showing a dependency on the vertex's degree and indicating that in all the networks highly connected vertexes tend to connect to poorly connected vertexes, i.e. they show a disassortative behavior. It is clear how this phenomenon is much more pronounced in trade than in migration, as can also be seen by the assortativity coefficientsAlso known as Pearson's degree correlation. in Table REF .
